scholarly journals Direct validation of dune instability theory

2021 ◽  
Vol 118 (17) ◽  
pp. e2024105118
Author(s):  
Ping Lü ◽  
Clément Narteau ◽  
Zhibao Dong ◽  
Philippe Claudin ◽  
Sébastien Rodriguez ◽  
...  
Keyword(s):  
Growth Rate ◽  
Large Scale ◽  
Maximum Growth ◽  
Theoretical Models ◽  
Periodic Pattern ◽  
Small Scale ◽  
Gobi Desert ◽  
Linear Regime ◽  
Sources Of Information ◽  
Instability Theory

Modern dune fields are valuable sources of information for the large-scale analysis of terrestrial and planetary environments and atmospheres, but their study relies on understanding the small-scale dynamics that constantly generate new dunes and reshape older ones. Here, we designed a landscape-scale experiment at the edge of the Gobi desert, China, to quantify the development of incipient dunes under the natural action of winds. High-resolution topographic data documenting 42 mo of bedform dynamics are examined to provide a spectral analysis of dune pattern formation. We identified two successive phases in the process of dune growth, from the initial flat sand bed to a meter-high periodic pattern. We focus on the initial phase, when the linear regime of dune instability applies, and measure the growth rate of dunes of different wavelengths. We identify the existence of a maximum growth rate, which readily explains the mechanism by which dunes select their size, leading to the prevalence of a 15-m wavelength pattern. We quantitatively compare our experimental results with the prediction of the dune instability theory using transport and flow parameters independently measured in the field. The remarkable agreement between theory and observations demonstrates that the linear regime of dune growth is permanently expressed on low-amplitude bed topography, before larger regular patterns and slip faces eventually emerge. Our experiment underpins existing theoretical models for the early development of eolian dunes, which can now be used to provide reliable insights into atmospheric and surface processes on Earth and other planetary bodies.

Filamentation of a laser beam in a strongly ionized magnetoplasma

1978 ◽  
Vol 19 (1) ◽  
pp. 55-61 ◽  
Author(s):  
L. A. Pitale
Keyword(s):  
Magnetic Field ◽  
Growth Rate ◽  
Laser Beam ◽  
Spatial Scale ◽  
Large Scale ◽  
Maximum Growth ◽  
Mean Free Path ◽  
Maximum Growth Rate ◽  
Main Beam ◽  
Small Scale

On a time-scale of the order of the energy relaxation time, a high power laser beam, propagating in a strongly ionized magnetoplasma is shown to be unstable for small scale fluctuations. In the domain r0 < [mi/m]½ λm. v2/[ω2c + v2] (r0, λm, v, ωc, and m being respectively the spatial scale of the perturbation, electron mean free path, collision frequency, cyclotron frequency and mass and mi being the ion mass) the main loss of excess electron energy is due to thermal conduction; in the other limit collisional loss dominates. It is shown that for small scale fluctuations the growth rate increases with (i) increasing magnetic field and (ii) increasing r0. For large scale fluctuations the magnetic field does not show any effect; the growth rate, however, diminishes with increasing spatial scale. A maximum growth rate is obtained both for some optimum value of scale length and for intensity of the main beam.

Drift wave driven zonal flows in electron–positron–ion plasmas

2010 ◽  
Vol 76 (3-4) ◽  
pp. 635-643 ◽  
Author(s):  
T. D. KALADZE ◽  
O. A. POKHOTELOV ◽  
M. SHAD
Keyword(s):  
Growth Rate ◽  
Large Scale ◽  
Maximum Growth ◽  
Finite Amplitude ◽  
Maximum Growth Rate ◽  
Small Scale ◽  
Drift Waves ◽  
Zonal Flows ◽  
Electron Positron ◽  
Spatial Dimensions

AbstractThe generation of large-scale zonal flows by small-scale electrostatic drift waves in electron–positron–ion (EPI) plasma is considered. The generation mechanism is based on the parametric excitation of convective cells by finite amplitude drift waves. To describe this process, the Hasegawa–Mima equation generalized for the case of EPI plasma is used. Explicit expressions for the maximum growth rate as well as for the optimal spatial dimensions of the zonal flows are obtained. Dependence of the growth rate on the spectrum purity of the wave packet is also investigated. The relevant instability conditions are determined.

Large-Scale Circulation in a Rectangular Enclosure With Periodic Boundary Temperature

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
D. Sitarski ◽  
R. J. Lee ◽  
J. R. Saylor ◽  
John P. McHugh
Keyword(s):  
Large Scale ◽  
Periodic Boundary ◽  
Periodic Pattern ◽  
Small Scale ◽  
Large Scale Circulation ◽  
Left Wall ◽  
Heating And Cooling ◽  
Rectangular Basin ◽  
Spatially Periodic ◽  
Interdigitated Array

An experiment in a rectangular basin of water is used to demonstrate that a large-scale circulation will result from a zero-mean thermal forcing. The thermal force is a spatially periodic pattern of heating and cooling at the top surface, achieved with an interdigitated array of hot and cold tubes. The experimental results show a very robust, steady flow with ascending flows at each end of the tank and a single descending jet near the left wall. These results suggest that small-scale forcing in surface-driven flows may result in significant large-scale subsurface motion.

scholarly journals The role of the sonic scale in the growth of magnetic field in compressible turbulence

2020 ◽  
Vol 493 (3) ◽  
pp. 4400-4408
Author(s):  
Itzhak Fouxon ◽  
Michael Mond
Keyword(s):  
Magnetic Field ◽  
Growth Rate ◽  
Large Scale ◽  
Compressible Turbulence ◽  
Small Scale ◽  
Integral Scale ◽  
Time Model ◽  
Turbulent Eddies ◽  
Field Amplification ◽  
The Magnetic Field

ABSTRACT We study the growth of small fluctuations of magnetic field in supersonic turbulence, the small-scale dynamo. The growth is due to the smallest and fastest turbulent eddies above the resistive scale. We observe that for supersonic turbulence these eddies are localized below the sonic scale ls, defined as the scale where the typical velocity of the turbulent eddies equals the speed of sound, and are therefore effectively incompressible. All previous studies have ignored the existence of the sonic scale and consequently treated the entire inertial range as made up of compressible eddies. However, at large Mach numbers ls is much smaller than the integral scale of the turbulence so the fastest growing mode of the magnetic field belongs to small-scale incompressible turbulence. We determine this mode and the associated growth rate numerically with the aid of a white noise in time model of turbulence whose approximate validity for the description of the Navier–Stokes turbulence is explained. For that purpose, we introduce a new non-dimensional number Rsm that we name the magnetosonic Reynolds number that describes the division of the magnetic field amplification range between small-scale incompressible eddies and large-scale supersonic ones. We show that indeed, as Rsm grows (which means that the incompressible eddies occupy a larger portion of the magnetic field amplification range) the growth rate of the fastest growing mode increases, while the spatial distribution of the growing magnetic field shifts to smaller scales. Our result implies the existence of small-scale dynamo for compressible homogeneous turbulence.

scholarly journals Moffatt-drift-driven large-scale dynamo due to  fluctuations with non-zero correlation times

2016 ◽  
Vol 798 ◽  
pp. 696-716 ◽  
Author(s):  
Nishant K. Singh
Keyword(s):  
Magnetic Field ◽  
Differential Equation ◽  
Growth Rate ◽  
White Noise ◽  
Large Scale ◽  
Maximum Growth ◽  
Dynamo Action ◽  
Integro Differential Equation ◽  
Large Scale Magnetic Field ◽  
Finite Memory

We present a theory of large-scale dynamo action in a turbulent flow that has stochastic, zero-mean fluctuations of the ${\it\alpha}$ parameter. Particularly interesting is the possibility of the growth of the mean magnetic field due to Moffatt drift, which is expected to be finite in a statistically anisotropic turbulence. We extend the Kraichnan–Moffatt model to explore effects of finite memory of ${\it\alpha}$ fluctuations, in a spirit similar to that of Sridhar & Singh (Mon. Not. R. Astron. Soc., vol. 445, 2014, pp. 3770–3787). Using the first-order smoothing approximation, we derive a linear integro-differential equation governing the dynamics of the large-scale magnetic field, which is non-perturbative in the ${\it\alpha}$-correlation time ${\it\tau}_{{\it\alpha}}$. We recover earlier results in the exactly solvable white-noise limit where the Moffatt drift does not contribute to the dynamo growth/decay. To study finite-memory effects, we reduce the integro-differential equation to a partial differential equation by assuming that ${\it\tau}_{{\it\alpha}}$ be small but non-zero and the large-scale magnetic field is slowly varying. We derive the dispersion relation and provide an explicit expression for the growth rate as a function of four independent parameters. When ${\it\tau}_{{\it\alpha}}\neq 0$, we find that: (i) in the absence of the Moffatt drift, but with finite Kraichnan diffusivity, only strong ${\it\alpha}$ fluctuations can enable a mean-field dynamo (this is qualitatively similar to the white-noise case); (ii) in the general case when also the Moffatt drift is non-zero, both weak and strong ${\it\alpha}$ fluctuations can lead to a large-scale dynamo; and (iii) there always exists a wavenumber ($k$) cutoff at some large $k$ beyond which the growth rate turns negative, irrespective of weak or strong ${\it\alpha}$ fluctuations. Thus we show that a finite Moffatt drift can always facilitate large-scale dynamo action if sufficiently strong, even in the case of weak ${\it\alpha}$ fluctuations, and the maximum growth occurs at intermediate wavenumbers.

scholarly journals Using an individual-based model with four trophic levels to model the effect of predation and competition on squid recruitment

2011 ◽  
Vol 69 (3) ◽  
pp. 439-447 ◽  
Author(s):  
Matteo Sinerchia ◽  
Anthony J. Field ◽  
John D. Woods ◽  
Silvana Vallerga ◽  
Wes R. Hinsley
Keyword(s):  
Growth Rate ◽  
Food Availability ◽  
Life Histories ◽  
Large Scale ◽  
Trophic Levels ◽  
Small Scale ◽  
Emergent Properties ◽  
Individual Based Model ◽  
Top Predators ◽  
Exogenous Forcing

Abstract Sinerchia, M., Field, A. J., Woods, J. D., Vallerga, S., and Hinsley, W. R. 2012. Using an individual-based model with four trophic levels to model the effect of predation and competition on squid recruitment. – ICES Journal of Marine Science, 69: 439–447. The Lagrangian Ensemble recruitment model (LERM) is the first prognostic model of fisheries recruitment based upon individuals. It incorporates five functional groups: phytoplankton (diatoms), herbivorous zooplankton (copepods), carnivorous zooplankton (squid paralarvae), and two top predators. Physiology and behaviour are described by equations derived from literature based on reproducible laboratory experiments. LERM is built using the Lagrangian Ensemble metamodel, in which the demography and biofeedback of each dynamic population are diagnostic properties, emerging from the life histories of individuals. The response of the plankton ecosystem and squid recruitment to different scenarios of exogenous forcing is investigated. Simulations were run at 41°N 27°W (Azores) under a stationary annual cycle of atmospheric forcing. The ecosystem adjusts to a stable attractor for each scenario. The emergent properties of each attractor are investigated, with focus on predation, competition for food, and spawning magnitude. Annual recruitment is a complex emergent property dependent on several factors, including food availability, predation, competition, and post-hatching growth rate, as proposed by Hjort's critical period theory, relating recruitment to predation mortality, depending on growth rate and hence food availability. The model provides a useful step towards linking small-scale processes governing the life histories of larvae and fisheries on the large scale.

The influence of atmospheric waves on the general circulation of the middle atmosphere

1987 ◽  
Vol 323 (1575) ◽  
pp. 693-705 ◽  
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Middle Atmosphere ◽  
Chemical Species ◽  
Theoretical Models ◽  
Irreversible Processes ◽  
Small Scale ◽  
Wind Fields ◽  
Meridional Transport ◽  
Radiative Balance

To a first approximation, the basic features of the globally averaged structure of the middle atmosphere (such as the warm stratopause and cold mesopause) can be understood on radiative grounds alone. However, dynamical processes must be invoked if the observed latitudinally varying structures of the zonal-mean temperature and wind fields are to be explained. Particularly large departures from a hypothetical radiatively determined state occur in the winter stratosphere (especially in the Northern Hemisphere) and in the upper mesosphere at the solstices. Simple theoretical models indicate that the primary dynamical mechanisms that drive the middle atmosphere away from radiative balance are wave motions, notably large-scale planetary waves and small-scale gravity waves. Much current research is being devoted to understanding the complex transient and irreversible processes by which such waves can influence the zonal-mean state and also lead to the meridional transport of chemical species.

scholarly journals Triad Instability of Planetary Rossby Waves

2007 ◽  
Vol 37 (8) ◽  
pp. 2158-2171 ◽  
Author(s):  
Yu Zhang ◽  
Joseph Pedlosky
Keyword(s):  
Growth Rate ◽  
Large Scale ◽  
Rossby Waves ◽  
Maximum Growth ◽  
Maximum Growth Rate ◽  
Coriolis Parameter ◽  
Planetary Scale ◽  
Baroclinic Waves ◽  
Eastern Boundary ◽  
Ray Path

Abstract The triad instability of the large-scale, first-mode, baroclinic Rossby waves is studied in the context of the planetary scale when the Coriolis parameter is to its lowest order varying with latitude. Accordingly, rather than remain constant as in quasigeostrophic theory, the deformation radius also changes with latitude, yielding new and interesting features to the propagation and triad instability processes. On the planetary scale, baroclinic waves vary their meridional wavenumbers along group velocity rays while they conserve both frequencies and zonal wavenumbers. The amplitudes of both barotropic and baroclinic waves would change with latitude along a ray path in the same way that the Coriolis parameter does if effects of the nonlinear interaction are ignored. The triad interaction for a specific triad is localized within a small latitudinal band where the resonance conditions are satisfied and quasigeostrophic theory is applicable locally. Using the growth rate from that theory as a measure, at each latitude along the ray path of the basic wave, a barotropic wave and a secondary baroclinic wave are picked up to form the most unstable triad and the distribution of this maximum growth rate is examined. It is found to increase southward under the assumption that triad interactions do not cause a noticeable decrease in the quantity of the basic wave’s amplitude divided by the Coriolis parameter. Different barotropic waves that maximize the growth rate at different latitudes have almost the same meridional length scale, on the order of the deformation radius. With many rays starting from different latitudes on the eastern boundary and with wavenumbers on each of them satisfying the no-normal-flow condition, the resulting two-dimensional distribution of the growth rate is a complicated function of the relative relations of zonal wavenumbers or frequencies on different rays and the orientation of the eastern boundary. In general, the growth rate is largest on rays originating to the north.

scholarly journals THE IMPACT OF THE ECONOMIC CRISIS IN THE IT&C INDUSTRY – EVIDENCE FROM BUCHAREST

2020 ◽  
Vol 4 (1) ◽  
Author(s):  
Claudia POPESCU ◽  
Alexandru GAVRIȘ
Keyword(s):  
Large Scale ◽  
Small And Medium Enterprises ◽  
Maximum Growth ◽  
Business Environment ◽  
Political Crisis ◽  
Small Scale ◽  
Industrial Growth ◽  
Almost Everywhere ◽  
Medium Enterprises ◽  
The Impact

Despite its peripheral location within the European metropolitan system, Bucharest has significant competitive advantages – large scale market, high-skilled labour pooling, dynamic business environment, institutional capacity, and knowledge organizations. The location of MNCs has enhanced the domestic small-scale entrepreneurship and the emergence of an innovative IT&C cluster. As the world economic crises stroke almost everywhere, in the case of Bucharest it was enhanced by a political crisis which diminished the industrial growth. In this context we focus on the IT small and medium enterprises which, by surviving the crises and developing even more, show clear evidence of strengthening the cluster identity. Based on the two-digit CANE data on employment, the paper analyzes in an empirical way the IT firms from Bucharest between two representative moments: 2007 the year of maximum growth for Romania and 2010. We attempt to identify the factors contributing to the growth of the cluster and to assess the contribution of the cluster to the generation of regional wealth and jobs. The results shows that, despite of the crises, the local entrepreneurship alongside the continuous foreign interest in the local workforce have pulled together an emerging industrial cluster.

Rossby-wave driven zonal flows in the ionospheric E-layer

2007 ◽  
Vol 73 (1) ◽  
pp. 131-140 ◽  
Author(s):  
T. D. KALADZE ◽  
D. J. WU ◽  
O. A. POKHOTELOV ◽  
R. Z. SAGDEEV ◽  
L. STENFLO ◽  
...  
Keyword(s):  
Large Scale ◽  
Rossby Waves ◽  
Maximum Growth ◽  
Present Theory ◽  
Finite Amplitude ◽  
Maximum Growth Rate ◽  
Small Scale ◽  
Reynolds Stresses ◽  
Zonal Flows ◽  
Coupled Equations

Abstract.A novel mechanism for the generation of large-scale zonal flows by small-scale Rossby waves in the Earth's ionospheric E-layer is considered. The generation mechanism is based on the parametric excitation of convective cells by finite amplitude magnetized Rossby waves. To describe this process a generalized Charney equation containing both vector and scalar (Korteweg–de Vries type) nonlinearities is used. The magnetized Rossby waves are supposed to have arbitrary wavelengths (as compared with the Rossby radius). A set of coupled equations describing the nonlinear interaction of magnetized Rossby waves and zonal flows is obtained. The generation of zonal flows is due to the Reynolds stresses produced by finite amplitude magnetized Rossby waves. It is found that the wave vector of the fastest growing mode is perpendicular to that of the magnetized Rossby pump wave. Explicit expression for the maximum growth rate as well as for the optimal spatial dimensions of the zonal flows are obtained. A comparison with existing results is carried out. The present theory can be used for the interpretation of the observations of Rossby-type waves in the Earth's ionosphere.

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